Wireless transmit receive units (WTRUs) in a UMTS Terrestrial Radio Access Network (UTRAN) can be in either of two modes: Idle or Connected. Based on WTRU mobility and activity while in connected mode, the UTRAN can direct the WTRU to transition between a number of radio resource control (RRC) sub-states: Cell_PCH, URA_PCH, Cell_DCH and Cell_FACH. The Cell_PCH and URA_PCH states are used to trigger cell updates on a paging channel (PCH) for a mobile WTRU. The Cell_DCH state is characterized by dedicated channels (DCHs) in both the uplink and the downlink. On the WTRU side, this corresponds to continuous transmission and reception and can be demanding on user power requirements. The Cell_FACH state is characterized by a forward access channel (FACH) and does not use dedicated channels, thus allowing better power consumption, at the expense of a lower uplink and downlink throughput.
Recent work by the standardization bodies has identified the possibility of using high speed downlink packet access (HSDPA) in Cell_FACH and Cell_PCH/URA PCH. HSDPA is a feature that was introduced in Release 5 of the 3rd Generation Partnership Project (3GPP) specifications to operate in Cell_DCH. HSDPA tries to make better use of the downlink shared capacity by using three key concepts: Adaptive Modulation and Coding (AMC), retransmissions using a Hybrid-ARQ (HARQ) scheme, and Node B scheduling, all operating at a very fast rate.
The following HSDPA characteristics are relevant:                a. Every WTRU having a HSDPA connection is assigned a high speed downlink scheduling channel (HS-DSCH) Radio Network Temporary Identifier (H-RNTI). This identifier is unique within a cell and assigned by the serving Radio Network Controller (RNC).        b. A WTRU is attached to a single serving cell (Node B).        c. The WTRU has to receive indication of the physical channel resources to use (HS-PDSCH info), as well as how to set up the HARQ processes and the HARQ memory.        
In 3GPP Release 6, HSDPA was only supported in Cell_DCH. Therefore, when moving out of Cell_DCH state, all HSDPA resources had to be cleared. However, with the introduction of enhanced Cell_FACH the WTRU may move from Cell_DCH to any other state and still support HS-DSCH reception, which may not require the WTRU to clear all HSDPA resources. Therefore, an additional condition was added to the variable HS_DSCH_RECEPTION associated with HS-DSCH reception in Cell_DCH. The HSDPA resources are only cleared when the requirements to perform HS-DSCH reception are not met and the WTRU is in Cell_DCH.
It is assumed that if a WTRU supports HSDPA in Release 7, it also supports HSDPA in Cell_FACH and in Cell_PCH/URA_PCH. HS-DSCH reception in Cell_FACH can be configured without HS-DSCH reception in Cell_PCH and URA_PCH. HS-DSCH reception in Cell_PCH and URA_PCH without the support of HS-DSCH reception in Cell_FACH is not supported. The WTRU support for these features is signaled by the WTRU to the network.
The network signals its support for enhanced Cell_FACH via the System Information Block (SIB5/5bis) to the WTRU. Two new information elements (IEs) have been introduced:                a. “HS-DSCH common system information” indicating the HSDPA reception is supported for Cell_FACH state; and b. “HS-DSCH paging system information”—indicating that HSDPA is supported for Cell_PCH/URA_PCH state.To indicate whether HS-DSCH reception is ongoing in Cell_FACH, two new Boolean variables have been introduced, which are used as flags by the WTRU radio resource control (RRC) controller:        a. HS_DSCH_RECEPTION_Cell_FACH_STATE: If TRUE indicates that HS_DSCH reception in Cell_FACH reception is ongoing; and        b. HS_DSCH_RECEPTION_OF_CCCH_OF_ENABLED: If TRUE indicates that HS-DSCH reception is enabled for CCCH. This variable is set to TRUE when the WTRU is using a common H-RNTI to receive HSDPA traffic, and is set to FALSE when the WTRU is not.However, no new variable has been defined for HS-DSCH reception indication in Cell_PCH or URA_PCH states. When in Cell_PCH or URA_PCH, and if the IE “HS-DSCH paging system information” is broadcasted, the WTRU sets up HSDPA reception.        
Scenarios can occur where the WTRU transitions from Cell_DCH to a state that does not support HSDPA. For instance, the WTRU could transition from Cell_DCH to Cell_FACH in a cell that does not support the Enhanced Cell_FACH feature, or the WTRU could transition from Cell_DCH to Cell_PCH in a cell that does not support HS-DSCH reception in Cell_PCH. In such scenarios the HSDPA resources are not cleared and the WTRU will continue to perform HS-DSCH reception procedures. This behavior is undesirable since the WTRU will waste resources and battery while attempting to receive on this channel. In addition, failure to clear HSDPA resources (i.e. clear IEs, release HARQ resources, performing enhanced high speed medium access channel (MAC-ehs) reset) is likely to cause problems when the WTRU resumes HS-DSCH reception in another state.
Similar problems occur in some scenarios of cell reselection. For instance, the WTRU may reselect from a cell that supports HS-DSCH in CELL to a cell that does not support the Enhanced Cell_FACH feature and the HSDPA resources are not cleared.